This invention relates to compensation circuitry in electrical signal sensing apparatus and, more particularly, to automatic compensation circuitry operative at the start of transmission of information in a communications system.
In data communications systems, and particularly facsimile communications systems, information to be transmitted from a sending terminal to a receiving terminal typically modulates a carrier signal of fixed frequency, either by amplitude modulation techniques or by frequency modulation techniques. At the receiving end of the communications channel, the signal is demodulated into a form usable by the receiving terminal. In order to faithfully reproduce at the receiving end the information transmitted over the communications channel, it is necessary for the demodulation circuitry in the receiver to have some fixed reference point. Because of inherent component value drifting due to aging and temperature variations, it is necessary to provide some means for adjusting the reference point to compensate for this drifting. To avoid the necessity of human intervention in the operation of the apparatus, it would be desirable to provide some means for the automatic adjustment of the reference point to compensate for component value drifting.